1. Field of the Invention
This invention relates generally to the electroless metal plating of a substrate to provide a wear resistant coating and, in particular, to the use of a nickel, cobalt, phosphorous electroless plating bath composition to plate aluminum articles with a specially correlated nickel, cobalt, phosphorous alloy wear resistant coating.
2. Description of Related Art
There are many materials of construction and each has properties which make it useful for certain applications. A property needed in many applications is wear resistance to provide a long operating life for the part made from the material and wear resistance may be defined as the ability of a material to withstand erosion or wearing away when the material is in moving contact with another material. Wear resistance is an important property for materials employed for such uses as tools and household appliances to industrial products like machine parts, pumps and gears.
The material of construction used for an application is normally chosen for properties such as strength, cost, weight and the ability to be formed into the desired product. In many applications however, this material does not have the required wear resistance and cannot be used and this problem has been researched extensively and wear resistant coatings have been developed to extend the usefulness of materials. In automobiles, for example, lightweight metals such as aluminum are extensively used in the manufacturing process to reduce the weight of the car to increase its fuel efficiency and meet environmental regulations. Aluminum, however, does not have the wear resistance of the heavier steel and would wear out faster and need to be replaced more frequently. This is not economically practical and coatings have been developed to increase the wear resistance of aluminum for use in automobiles and other applications.
A coating for aluminum now used in automobiles is a nickel-Teflon deposit applied from an electroless nickel-Teflon metal plating bath. These coatings are functional alternatives to the use of heavier weight materials of construction and hard chromium deposits which are environmentally undesirable. Unfortunately, the demands of industry are continually increasing and more severe operating conditions, cost factors and environmental concerns dictate the need for materials, especially lightweight materials, which are more wear resistant and preferably more cost effective than existing materials and/or coatings now used.
The following discussion for convenience will be directed to the plating of aluminum and it will be understood to those skilled in the art, that similar properties are needed for other materials in other applications requiring wear resistant coatings. In general, new requirements to be met for wear resistant coatings on aluminum include a low heat treatment temperature preferably about 200.degree. C. or below for increasing the hardness and wear resistance of the plated coating, a hardness after heat treatment above approximately 600 and preferably 700-800 HV.sub.10 and the passing of a standard wear test such as the Taber Wear Index (TWI) which is calculated using a Taber Abraser. The Taber Abraser is an instrument designed to evaluate the resistance of surfaces to rubbing abrasion. The characteristic rub-wear action of the Abraser is produced by the contact of a test sample, turning on a vertical axis, against the sliding rotation of two abrading wheels. The wheels are driven by the sample in opposite directions about a horizontal axis displaced tangentially from the axis of the sample with one abrading wheel rubbing the specimen outward toward the periphery and the other, inward toward the center. The resulting abrasion marks form a pattern of crossed arcs over an area of approximately 30 square centimeters. Test results are expressed as a wear factor or numerical abrasion index of the test specimen and one method of evaluation is the TWI (rate of wear) based on the loss in weight in milligrams (mgs.) per thousand cycles of abrasion under a specific set of conditiions. The lower the TWI, the better the abrasion resistance quality of the material. Using a CS-10 Calibrase wheel under a load of 1 kilogram for 5000 cycles, TWI's less than 20 and preferably less than 10 are desired for aluminum and other materials to meet the necessary wear resistance requirements.
U.K. Patent No. 2272959 provides hard wearing surfaces for aluminum piston grooves having a hardness in the range of 300 to 750 HV by applying a coating of cobalt by electroless plating. The coating may contain 1-10% by weight phosphorous and may be heat treated after coating at a temperature between 150.degree. C. and 500.degree. C. for at least 30 minutes. The deposition of a wear resistant nickel-cobalt-phosphorous alloy is not disclosed. U.S. Pat. No. 4,983,428 provides wear resistant nickel-boron coatings on turbine engine parts using an electroless nickel plating bath containing ethylenethiourea.
Nickel-cobalt-phosphorous electroless metal plating baths are known to be used to form high coercive force films on substrates used for magnetic storage devices. "Electroless Plating Variables and Coercive Force of Nickel-Cobalt-Phosphorous Films" by G. W. Lawless and R. D. Fisher, Plating, June 1967, pages 709-713 shows the effect of solution composition variables such as Ni/Co ratio on the coercivity of Ni--Co--P films. Similarly, "Magnetic Properties of Electroless Cobalt Based Alloys" by F. Pearlstein and R. F. Weightman, Plating, June 1967, pages 714-716 shows the effect on magnetic properties of the coating by adding nickel sulfate to an electroless cobalt solution. U.S. Pat. No. 4,150,172 discloses an electroless bath containing cobalt ions, citrate ions, hypophosphite ions, phosphate ions and, if desired, nickel ions, for use to form magnetic recording film.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to enhance the wear resistance of a substrate by applying to the surface of the substrate from an electroless metal plating bath a specially defined nickel, cobalt, phosphorous alloy coating.
It is another object of the present invention to provide an aqueous composition for electrolessly plating a substrate with an enhanced wear resistant nickel, cobalt, phosphorous alloy coating.
A further object of the invention is to provide a process for plating a substrate with an electrolessly deposited nickel, cobalt, phosphorous alloy to enhance the wear resistance of the substrate.
It is yet another object of the invention to provide articles, especially aluminum articles, having a wear resistant coating of a specially defined electroless nickel, cobalt, phosphorous alloy.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.